Field Deployment Report: Bottom-Mounted ADCP at Cabo San Pablo

Deployment Notes: Cabo San Pablo, October 2023

The salt spray was blinding by 05:00 hours as we fought the surge to get the gear in the water. Cabo San Pablo isn't a place for the faint of heart; it's a chaotic intersection where steep bathymetric gradients collide with seasonal wind forcing. As I looked over the side of the vessel, the water wasn't blue—it was a churning, opaque brown. The energy here is visceral. You can feel the vibration of the incoming swell hitting the shoreline, compressing the water mass and accelerating currents into high-velocity jets that would make any oceanographer sweat.

The conditions were typical for a late-season transition. We had a stiff breeze pushing against a flood tide, creating a surface state that looked more like a washing machine than a coastal waterway. The water was thick with suspended sediment, a direct result of the tidal asymmetry that defines this stretch of coast. In Cabo San Pablo, the flood tide hits with a violence that the ebb simply cannot match, scouring the seabed and tossing coarse sand into the water column. This isn't just a 'turbid' environment. It's an acoustic nightmare.

What We Found

The data coming back was staggering. We recorded velocity shears that would rip a standard mooring right out of the sand. At some depths, the surface currents were screaming eastward, but just a few meters down, the water was practically stagnant or even creeping westward. This vertical decoupling is the real story of Cabo San Pablo. If you're using a single-point current meter, you're lying to yourself. You get a fragmented snapshot that misses the massive, dangerous movement of the water mass beneath the surface. We saw rapid velocity variability that shifted the vector by nearly 180 degrees in a matter of hours (likely driven by the wind-forcing override).

The most surprising part? The bin contamination. Because the water column is so unstable, the acoustic returns were messy. We saw significant side-lobe interference where the ADCP picked up signals from directions it wasn't actually pointing at. This happens when the sediment load gets too high; the signal bounces off the debris rather than the water's movement. It took a few days of filtering to get a clean signal, but once we did, the magnitude of the sediment transport became clear. The seabed geometry here acts as a natural funnel, amplifying every surge and trapping organic debris in a cycle of constant suspension.

Equipment Performance

We ditched the vessel-mounted units and went with a bottom-mounted configuration. A towed fish is useless here; the surface turbulence is too violent to maintain a steady heading, and you end up with more noise than data. I opted for a 300kHz ADCP instead of the 600kHz. Why? Because we needed the penetration. The 600kHz unit is great for high resolution in clear water, but in the grit of Cabo San Pablo, it struggles. The 300kHz gave us a more reliable profile through the sediment-heavy lower layers. Still, the benthos is rugged. We spent hours ensuring the tripod was seated firmly in the substrate to avoid tilt errors. Honestly, the instrument survived the environment, but the data cleaning process was a slog. We had to perform several sanity checks against local tide gauges to ensure the vertical velocity wasn't just an artifact of the instrument swaying in the surge.

Recommendations for Future Deployments

If you're heading back to this site, don't trust the mean sea level data—the spring-neap cycle creates deceptive imbalances that will throw off your timing. Focus on the vertical profile, not the surface.

• Frequency Choice: Stick to 300kHz. The 600kHz units suffer too much attenuation in these sediment-heavy surges.
• Mooring Weight: Over-engineer your bottom mount. Use heavy-duty tripod bases with deep spikes to prevent shifting during peak flood tides.
• Sampling Interval: Set a high temporal resolution (minutes, not hours). The velocity shifts here happen too fast for coarse averaging.
• Ground-Truthing: Deploy a secondary CTD string to map the salinity gradient; it helps separate the salt wedge dynamics from the wind-driven currents.

Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience in high-energy coastal environments.